1. Field of the Invention
The present invention generally relates to an image data processing system, and in particular to method and system which extracts a target object from an image.
2. Description of the Related Art
There have been proposed several object extraction techniques, for instance, using necessary information which is previously registered, using well-known focusing technology, and using plural image pick-up devices which are arranged in predetermined positional relationship.
According to the technique using the prepared necessary information, when no information can be prepared, the target object cannot be extracted. Further, an expensive distance-measuring device is needed to measure a distance from the principal point in the object side to a part of the object. Furthermore, a distance from the principal point in the image side to the image can be measured using auto-focusing mechanism, but the auto-focusing cannot always operate stably depending on illumination conditions, resulting in reduced reliable measurement.
According to the technique using the focusing technology, an image pickup device images a target object while varying the focal length. Therefore, it is necessary to place the target object within the focal depth.
The technique using plural image pickup devices has been disclosed in Japanese Patent Unexamined Publication No. 10-98646. According to this conventional technique, the target object can be extracted using the three-dimensional coordinates of the contour of the target object. The three-dimensional coordinates of the target object are obtained on the principal of triangulation by using two-dimensional feature points and their point correspondence between the images which are obtained by two image pickup devices, respectively. The technique can provide stable operation without the need of information regarding the target object and with little influence of illumination conditions.
However, the precision of target extraction largely depends on a threshold used to discriminate between the target object and the background and it is difficult to properly determine such a threshold. According the Publication (No. 10-98646), the threshold is determined through human eyes and further the system is not designed to extract two or more target objects but a single object.